Passive back-flushing thermal energy system

ABSTRACT

This invention provides a thermal energy system comprising a heat exchanger for transferring thermal energy between a source and a load, the heat exchanger having a primary side associated with the source, and a secondary side for conducting a fluid associated with the load, wherein the secondary side of the heat exchanger is passively back-flushed upon consumption of a portion of the fluid. Passive back-flushing prevents fouling of the heat exchanger due to sediments, scale, and mineral deposits which may be present in the circulating fluid.

RELATED APPLICATION

This application claims the benefit of the filing date of U.S. PatentApplication No. 60/311,095, filed Aug. 10, 2001, the contents of whichare incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

This invention relates to thermal energy systems employing heatexchangers. In particular, the invention relates to a thermal energysystem with passive back-flushing of a heat exchanger, and methods forpassively back flushing systems.

BACKGROUND OF THE INVENTION

Thermal energy systems incorporating heat exchangers typically comprisea primary loop, from which heat is supplied or removed, and a secondaryloop, to or from which heat is transferred. The heat exchanger transfersheat between the primary and the secondary loop. A heat transfer fluidis circulated through the primary loop, supplying heat to, or removingheat from, the primary side of the heat exchanger. A secondary fluid towhich heat is supplied or from which heat is removed flows through thesecondary side of the heat exchanger. The primary and secondary sides ofthe heat exchanger typically have numerous small passageways in closeassociation through which the fluids flow, which facilitate the transferof thermal energy therebetween.

Modern heat exchangers are compact and offer high performance, i.e.,high rates of heat transfer. High performance is usually achieved bymaking the passageways very small, and providing many of them. However,as the size of the passageways is reduced, they become more prone tofouling or complete blockage due to the accumulation of sediments,scale, and mineral deposits that may be present in the circulatingfluid. Fouling of the heat exchanger leads to a substantial drop inperformance of the system. Specific measures taken to minimize foulinginclude monitoring and control of the chemical composition of thefluids, frequent disassembly for cleaning of the flow passages, andoversizing of heat transfer surfaces and flow passages to ensure thatthey will have sufficient capacity even when operating at decreasedeffectiveness due to fouling. In the case of thermal systems for heatingpotable or process water, there is a high probability that mineral saltsand other impurities may be present in the water. In such cases apotential for fouling of the heat exchanger exists if the exchanger isnot routinely cleaned or flushed of accumulated matter. In manyapplications, such as residential and small commercial installations,monitoring of the chemical composition of the water, routine disassemblyand cleaning of the heat exchanger, or oversizing are not practical dueto the associated costs.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a thermalenergy system, comprising: a heat exchanger for transferring thermalenergy between a source and a load, the heat exchanger having a primaryside associated with said source, and a secondary side for conducting afluid associated with said load; wherein the secondary side of the heatexchanger is passively back-flushed upon consumption of a portion ofsaid fluid. In certain embodiments, the thermal energy system furthercomprises a storage tank associated with the load.

In one embodiment, the load is a hot water supply and the fluid iswater. In another embodiment, the load is a chilled water supply and thefluid is water.

In another embodiment, a thermal energy system of the invention furthercomprises a back-flushing valve, wherein the back-flushing valvepassively controls back-flushing of the secondary side of the heatexchanger. In certain embodiments, the back-flushing valve is activatedby at least one of flow rate, temperature, and pressure of the fluid. Ina preferred embodiment, the back-flushing valve is activated by flowrate of the fluid. In further embodiments, the back-flushing valveprovides a bypass flow when the valve is closed. In some embodiments thebypass flow is about 1% to about 20% of a flow rate during consumptionof the fluid.

In certain embodiments, the source is a heat source selected from solarheat, waste heat, geothermal heat, industrial process heat, a heat pump,a boiler, and a furnace. In a preferred embodiment, the heat source issolar heat.

In a further embodiment of the invention there is provided a thermalenergy system comprising: a heat exchanger for transferring thermalenergy between a source and a load, the heat exchanger having a primaryside associated with said source, and a secondary side for receivingfluid to be heated or cooled and outputting said heated or cooled fluid,the fluid flowing through the secondary side of the heat exchanger in afirst direction; an input for receiving mains fluid; and a back-flushingvalve for controlling flow of the heated or cooled fluid and the mainsfluid; wherein, upon consumption of a portion of the heated or cooledfluid, the back-flushing valve passively directs mains fluid through thesecondary side of the heat exchanger in a second direction opposite tothat traveled by the heated or cooled fluid. In one embodiment, theback-flushing valve provides a bypass flow when the valve is closed. Insome embodiments the bypass flow is about 1% to about 20% of a flow rateduring consumption of the fluid.

According to a further aspect of the invention there is provided amodule for a thermal energy system including a storage tank associatedwith a load, said module comprising: a heat exchanger for transferringheat from a heat source to a load, the heat exchanger having a primaryside for receiving heat from a heat source and a secondary side forreceiving water to be heated and outputting said heated water to theload, the heated water flowing through the secondary side of the heatexchanger in a first direction; an input for receiving mains water; anda back-flushing valve for controlling flow of the water to be heated andthe mains water; wherein, upon consumption of a portion of the water tobe heated, the back-flushing valve passively directs mains water throughthe secondary side of the heat exchanger in a second direction oppositeto that traveled by the water to be heated. In one embodiment, theback-flushing valve provides a bypass flow when the valve is closed. Insome embodiments the bypass flow is about 1% to about 20% of a flow rateduring consumption of the fluid.

By another aspect of the invention there is provided a method forpassively back flushing a heat exchanger in a thermal energy system,comprising: providing a heat exchanger for transferring thermal energybetween a source and a load, the heat exchanger having a primary sideassociated with said source, and a secondary side for conducting a fluidassociated with said load; providing a source of excess fluid; flowingthe fluid through the secondary side of the heat exchanger in a firstdirection; and upon consumption of at least a portion of the fluid bythe load, passively flowing said excess fluid through the secondary sideof the heat exchanger in a second direction opposite to the firstdirection.

In one embodiment of the method, the thermal energy system is a hotwater system. In certain embodiments, the heat source is selected fromsolar heat, waste heat, geothermal heat, industrial process heat, a heatpump, a boiler, and a furnace. In a preferred embodiment, the heatsource is solar heat. In yet another embodiment of the method, thethermal energy system is a chilled water system.

In one embodiment, the back-flushing step is activated by at least oneof flow rate, temperature, and pressure of the fluid. In a preferredembodiment, the back-flushing step is activated by flow rate of thefluid.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are described below, by way of example,with reference to the accompanying drawings, wherein:

FIG. 1 is a block diagram of a generalized thermal energy heating systemin accordance with the invention, shown in charging mode;

FIG. 2 is a block diagram of a generalized thermal energy heating systemin accordance with the invention, shown in back-flushing mode;

FIG. 3 is a block diagram of a generalized thermal energy cooling systemin accordance with the invention, shown in charging mode;

FIG. 4 is a block diagram of a generalized thermal energy cooling systemin accordance with the invention, shown in back-flushing mode;

FIG. 5 is a schematic diagram of a back-flushing valve according to theinvention; and

FIG. 6 is a schematic diagram of a back-flushing valve according to theinvention.

FIG. 7 is a schematic diagram of another embodiment of a back-flushingvalve according to the invention.

FIG. 8 is a schematic diagram of a thermal energy system according tothe invention, with temperature sensors to measure fluid temperaturesduring operation. Arrows indicate flow direction during back-flushing.

FIG. 9 is a plot showing water temperatures obtained with the system ofFIG. 8, prior to, during, and after a 3-minute draw.

DETAILED DESCRIPTION OF THE INVENTION

In accordance with the invention, a heat exchanger has a primary side,to which heat is supplied, or from which heat is removed, and asecondary side, from which heat is obtained, or to which heat is lost. Aheat or cooling source can be either in direct contact with the primaryside of the heat exchanger, or located away from the heat exchanger.Examples of a heat source are solar heat, waste heat (e.g., exhaust),geothermal heat, industrial process heat, a heat pump, a boiler, and afurnace. Examples of a cooling source are a chiller (e.g., refrigerationdevice) and a geothermal source. Where the source is located away fromthe heat exchanger, the source can be coupled to the heat exchanger viaa heat transfer fluid. The heat transfer fluid flows through a pluralityof channels in the primary side of the heat exchanger, those channelsbeing closely associated with a plurality of channels in the secondaryside of the heat exchanger. The heat transfer fluid can be, for example,water (which may be purified, e.g., distilled, or waste water, e.g.,water from an industrial process), an antifreeze solution (e.g.,propylene glycol), steam, refrigerant, exhaust gas, oil, and the like.In some embodiments, the primary side of the heat exchanger comprises anopen loop, wherein the heat transfer fluid (e.g., exhaust gas) is simplyreleased after passing through the primary side of the heat exchanger.In other embodiments, the primary side of the heat exchanger comprises aclosed loop, wherein the heat transfer fluid is retained in the systemand circulated between the heat/cooling source and the primary side ofthe heat exchanger. In systems with an open primary loop, fouling of theprimary side of the heat exchanger due to impurities in the heattransfer fluid can be mitigated by, for example, over-sizing the heatexchanger. In systems with a closed primary loop, fouling of the primaryside of the heat exchanger can be mitigated by controlling the chemicalcomposition of the heat transfer fluid to remove any impurities.

The secondary side of the heat exchanger is also prone to fouling, andcan be the most important factor that degrades system performance.Fouling of the secondary side of the heat exchanger is most common inapplications where the fluid in the secondary loop is consumed, and mustbe replenished. Examples of such systems are potable hot or chilledwater supplies, and industrial processes requiring a heated or chilledfluid such as water, where the water is consumed in the process. In bothof these examples, water enters the system from a source, and carrieswith it impurities (e.g., sediment, minerals, salts, and other solutes)that lead to fouling of the heat exchanger. Although water entering thesystem can be pre-treated (e.g., filtered) to remove impurities, suchpre-treatment is not practical in residential and small commercialapplications. Thus, thermal energy systems for storing heat as hotwater, and/or for supplying potable hot water, such as, for example,solar water heating systems, heat pump systems, and district heatingsystems, in both residential and commercial installations, aresusceptible to heat exchanger fouling.

According to one aspect of the invention there is provided a thermalenergy system comprising a heat exchanger, wherein the secondary side ofthe heat exchanger is passively back-flushed. In preferred embodiments,the secondary side of the heat exchanger is passively back-flushed inresponse to a change in one or more variables (e.g., temperature,pressure, flow rate) of the fluid in the secondary side of the heatexchanger. The invention is particularly suited to applications wherethe fluid to which heat is supplied or from which heat is removed isconsumed, such as, for example, water heating or cooling systems. Insuch systems, the secondary side of the heat exchanger is passivelyback-flushed each time the system is replenished with fluid. Accordingto the invention, passive back-flushing of the heat exchanger is anormal operation of the system, and does not require user-interventionor external controls to operate. Back-flushing can routinely beperformed many times within a short period (e.g., one day). Duringback-flushing, scale, mineral deposits, and sediment are flushed out ofthe heat exchanger, thus preventing fouling of the heat exchanger.

As used herein, the term “thermal energy” is a term of art and isunderstood to encompass both hot and cold.

It will be appreciated that the invention is not limited to heating orcooling water. Thus, while the invention is described herein primarilywith respect to use with water, it can be used with other fluids.

It will also be appreciated that, although the invention is describedherein primarily with respect to thermal energy systems, the inventionis not limited thereto. The invention is suitable for use otherapplications, such as a system employing a fluid medium which isconsumed and occasionally replenished, where passive back-flushing of atleast a portion of the system (e.g., a filter) with the replenishingfluid is beneficial.

In one embodiment, a thermal energy system according to the invention isa water heating system, for example, for supplying domestic potable hotwater. As shown in the embodiment of FIG. 1, the primary loop is aclosed loop comprising a heat source 2 and suitable tubing or pipes 6for circulating a heat transfer fluid between the heat source 2 and theprimary side of a heat exchanger 4. A pump 18 can optionally be insertedinto the primary loop to facilitate circulation of the heat transferfluid. In some embodiments, such as those employing a water-based heattransfer fluid (e.g., a water-propylene glycol solution), the primaryloop additionally comprises an expansion tank (not shown), to compensatefor expansion/contraction of the heat transfer fluid as it changestemperature. The secondary loop of the system comprises the secondaryside of the heat exchanger 4, a water storage tank 8, a “T” joint 10,and a back-flushing control valve 12. Hot water is drawn from the top ofthe water storage tank 8 via a pipe or tube 16. Mains water enters thesecondary loop via the “T” joint 10, to replenish water drawn from thesecondary loop via a pipe 16.

As used herein, the term “mains water” refers to water entering thesystem from a water source, such as a city water distribution network ora well. Mains water enters the secondary loop of the system to be heatedor chilled.

The embodiment of FIG. 1 is shown in charging mode; that is, water isnot being drawn from the system via pipe 16, and water is not enteringthe secondary loop via the “T” joint 10. In charging mode the directionof flow of fluid (in this example, water) is indicated by arrows inFIG. 1. Operation of this embodiment is as follows. When water is notbeing drawn from the storage tank 8, the back-flushing control valve 12is open. Water circulates through the secondary side of the heatexchanger 4 and the storage tank 8 such that heated water flows upwardthrough the valve 12 and into the top of the tank, and out of the bottomof the tank into the heat exchanger. This flow can be established by apump, or by convection, and stratifies the storage tank 8, such that thehottest water is at the top of the tank. Preferably the flow isestablished by convection, which simplifies the system design andfacilitates back-flushing of the secondary side of the heat exchanger(described below). Also, it is preferred that this flow rate is slow(e.g., up to about 2.5 L/min), to avoid mixing of water in the storagetank 8.

When water is drawn from the storage tank 8, the back-flushing controlvalve 12 passively closes. As shown in FIG. 2, closing of the valve 12causes mains water to be routed through the secondary side of the heatexchanger 4 and enters the storage tank 8 at the bottom. Moreover, theflow of mains water through the secondary side of the heat exchanger 4is in the opposite direction to the flow of water during heating (whenmains water is not being drawn into the system). This opposite directionof flow provides passive back-flushing of the secondary side of the heatexchanger 4. Thus, each time water is drawn from the storage tank 8, thesecondary side of the heat exchanger 4 is passively back-flushed.

Many of the fouling components in water are solutes (e.g., inversesoluble salts such as CaCO₃, Mg(OH)₂, CaSiO₃, and CaSO₄) thatprecipitate at high temperatures (Purdy et al., Conference Proceedings,Solar Energy Society of Canada Inc., Montreal, 1998). By back-flushingthe heat exchanger 4 with unheated mains water, solutes precipitated inthe secondary side of the heat exchanger 4 are put back into solution.The solutes are deposited in the storage tank 8, where they areprecipitated, with little impact on the system, and/or eventuallydischarged from the tank as hot water is drawn therefrom.

In another embodiment, a thermal energy system according to theinvention is a water cooling system, for example, for supplying domesticpotable chilled water. As shown in FIG. 3, the primary loop, which canbe an open loop or a closed loop, comprises a cooling source or chiller17 and suitable tubing or pipes 6 for circulating a heat transfer fluidbetween the chiller 17 and the primary side of a heat exchanger 4. Apump 18 can optionally be inserted into the primary loop to facilitatecirculation of the heat transfer fluid. In some embodiments, such asthose employing a water-based heat transfer fluid (e.g., awater-propylene glycol solution), the primary loop additionallycomprises an expansion tank (not shown), to compensate forexpansion/contraction of the heat transfer fluid as it changestemperature. The secondary loop of the system comprises the secondaryside of the heat exchanger 4, a water storage tank 8, a “T” joint 10,and a back-flushing control valve 12. Cold water is drawn from thebottom of the storage tank 8 via a pipe or tube. Mains water enters thesecondary loop via the “T” joint 10, to replenish water drawn from thesecondary loop.

The embodiment of FIG. 3 is shown in charging mode; that is, water isnot being drawn from the system via pipe 18, and water is not enteringthe secondary loop via “T” joint 10. In charging mode the direction offlow of fluid (in this example, water) is indicated by arrows in FIG. 3.Operation of this embodiment is as follows. When water is not beingdrawn from the storage tank 8, the back-flushing control valve 12 isopen. Water circulates through the secondary side of the heat exchanger4 and the storage tank 8 such that chilled water flows downward throughthe valve 12 and into the bottom of the tank, and out of the top of thetank into the heat exchanger. This flow can be established by a pump, orby convection, and stratifies the storage tank 8, such that the coldestwater is at the bottom of the tank. Preferably the flow is establishedby convection, which simplifies the system design and facilitatesback-flushing of the secondary side of the heat exchanger (describedbelow). Also, it is preferred that this flow rate is slow (e.g., up toabout 2.5 L/min), to avoid mixing of water in the storage tank 8.

When water is drawn from the storage tank 8, the back-flushing controlvalve 12 passively closes. As shown in FIG. 4, closing of the valve 12causes mains water to be routed through the secondary side of the heatexchanger 4 and enters the storage tank 8 at the top. Moreover, the flowof mains water through the secondary side of the heat exchanger 4 is inthe opposite direction to the flow of water during chilling (when mainswater is not being drawn into the system). This opposite direction offlow provides passive back-flushing of the secondary side of the heatexchanger 4. Thus, each time water is drawn from the storage tank 8, thesecondary side of the heat exchanger 4 is passively back-flushed.

In a preferred embodiment, a thermal energy system according to theinvention is a solar hot water system. The hot water system is much likethe generalized system shown in FIG. 1, and further comprises one ormore solar collectors as a heat source 2. Heat transfer fluid is heatedin the solar collector(s) and the heat is transferred to water in theheat exchanger 4.

In a more preferred embodiment, the storage tank 8 is any standard,commercially available domestic hot water tank (e.g., 180 to 450liters), and various components of the system (e.g., the heat exchanger,the back-flushing control valve) are adapted to attach to such hot watertank. In a further embodiment, shown by example in Appendix 1, theinvention provides a module adapted for ease of installation on anexisting hot water tank, the module comprising a heat exchanger, aback-flushing control valve, and optionally further components such asone or more circulating pumps, one or more expansion tanks for the heattransfer fluid, and an electronic interface. The optional electronicinterface can provide information regarding system performance, forexample. When used with a standard hot water tank, a system according tothe invention supplements the hot water tank, and thus reduces theenergy cost of heating water.

To maximize effectiveness of the heat exchanger and to improve overallsystem efficiency, flow rates in the primary and secondary sides of theheat exchanger should be of similar magnitude. The heating or coolingsource is an influencing factor in determining flow rate in the primaryand secondary loops. However, to maximize overall efficiency of a systememploying a storage tank for domestic hot water, such as that describedin the above embodiment, stratification of the tank, with, e.g., hottestwater at the top, a slow flow rate is necessary.

For example (see also Appendix 1), in a preferred embodiment of theinvention, there is provided a solar hot water system optimized for atypical residential application, e.g., a North American household (up tofive individuals) with hot water consumption of about 200 to 300 L/day,and a 270 L hot water tank. With an average of 8 hours (480 minutes) ofheating per day, and about 3 to 6 m² of solar collector area, it wouldtake a flow rate of about 0.6 L/min to charge the hot water tank. Insuch installations, a heat exchanger with a total heat exchange area ofabout 0.25 m² to about 1 m² would be appropriate. For example, astainless-steel, brazed plate heat exchanger, model no. E8-20, availablefrom SWEP International of Sweden, is suitable.

The preferred embodiment of the invention thus utilizes fluid flow ratesthrough the heat exchanger which are slower than those used inconventional high-flow designs. According to the invention the flow rateof heat transfer fluid is in the range of about 0.5 to about 2.5 L/min,preferably about 0.5 L/min to about 1.5 L/min. The flow rate of fluidthrough the secondary side of the heat exchanger is from 0 L/min toabout 2.5 L/min. The maximum flow rate through the secondary side of theheat exchanger occurs when the water in the storage tank is cold (i.e.,uncharged), and the flow rate gradually slows as the water in the tankheats up. The convective flow essentially stops when the tank is fullycharged, i.e, when the temperature in the storage tank is approximatelythe same as that of the primary loop. For example, in certainembodiments, convective flow stops when the water in the tank reachesabout 60° C. In this regard the system is self limiting with respect tothe maximum temperature reached by water in the tank.

It will be appreciated that a thermal energy system according to theinvention can be scaled appropriately for larger or smallerinstallations, and for applications other than solar hot water heating.

In the generalized embodiment shown in FIG. 1, and in the solar hotwater system described above, the back-flushing control valve isnormally open during heating of the fluid in the storage tank, andclosed when fluid is drawn therefrom, so that the secondary side of theheat exchanger is back-flushed. The back-flushing control valve can beany valve that is activated (e.g., opens, closes) by a change in one ormore system variables (e.g., temperature, pressure, flow rate). It ispreferable that the back-flushing control valve is passive, meaning thatactivation of the valve does not require user intervention and isautomatic. It is also preferable that activation of the valve is notscheduled. In one embodiment, the back-flushing control valve isthermally activated (i.e., activated by a change in fluid temperature).For example, where the system is for heating potable water, the valve isopen when the water is warm (e.g., about 15° C. or warmer, preferablyabout 25° C. or warmer), and closed for lower water temperatures. Ofcourse, these valve opening and closing temperatures depend on thetemperature of the mains water. In this example, drawing hot water fromthe storage tank causes unheated mains water to flow into the system,and when the mains water reaches the thermally-activated valve, thevalve closes, forcing mains water to flow through and back-flush thesecondary side of the heat exchanger.

In accordance with another aspect of the invention, there is provided aback-flushing control valve for a thermal energy system. In a preferredembodiment, the back-flushing control valve is a specialized ball valveactivated by the flow rate of fluid therethrough. In general, theback-flushing control valve comprises a valve body having an input port,an output port, and valve seat, an orifice passing through the valveseat, and a ball for engaging the valve seat. In one embodiment, shownin FIG. 5, the valve is designed for vertical orientation, with theoutput port 24 and valve seat 26 facing upwards, and orifice 32 passingthrough the valve seat 26. The ball 28 is disposed in a cavity 30between the input port 22 and seat 26. Preferably, the cavity has aninside diameter of approximately the same magnitude as that of the pipeto which it is connected, so as to avoid turbulence in the flow.Although not shown in FIG. 5, it will be appreciated that the ball 28can be captured (retained) in the cavity 30 by providing a suitable bossor the like, or a retaining screw (see, for example. FIG. 7) within thecavity 30. It should be noted that although the ball 28 is depicted inFIG. 5 as generally spherical, other shapes, (e.g., oblong oregg-shaped) are contemplated by the invention. The valve body can bemade from any suitable material such as plastics, brass, copper, bronze,etc. The input port 22 and output port 24 are adapted for connection,via screw threads, soldering, or the like, to standard pipes orconnectors.

In a valve optimized for use with a solar hot water system as describedabove, the ball 28 can move freely within the cavity 30, but generallysinks at flow rates below about 2.5 L/min, so that when the flow ratethrough the valve is below about 2.5 L/min, the ball will not engage thevalve seat 26. However, at flow rates above about 2.5 L/min, such asthose achieved when mains water enters the system upon drawing waterfrom the storage tank, the ball rises and engages the valve seat 26.Thus, in the thermal energy system of the invention, the valve permitsconvective flow through the heat exchanger up to about 2.5 L/min duringheating. When hot water is drawn from the storage tank, mains waterenters the system at a higher flow rate, and such higher flow ratecloses the valve. When the valve is closed, mains water is routedthrough the secondary side of the heat exchanger, thereby back-flushingthe heat exchanger. It will of course be appreciated that the flow ratesdiscussed above are exemplary, and are based on a convective flow rateup to about 2.5 L/min in a domestic hot water system. The system/valvecan be adapted for other flow rates as considered below.

Factors affecting the flow rate at which the valve closes include thediameter of the cavity of the valve, the diameter of the orifice in thevalve seat, the diameter/size of the ball, the density of the ball, andthe presence/absence of air bubbles adhered to the ball. It is preferredthat the ball provide the same closure flow rate with and without airbubbles. For example, when the density of the ball is selected so thatthe valve closes at a flow rate of about 1.5 L/min (with air bubbles),the valve closes at the flow rates indicated in Table 1 with no airbubbles adhered to the ball. TABLE 1 Effect of Ball Diameter and AirBubbles on Valve Closure Flow Rate Ball Diameter (mm) Closure Flow Rate(L/min) Difference in (5 mm < cavity Without air Closure Flow diameter)With air bubbles bubbles Rate (L/min) 8 1.5 3.1 1.6 9 1.5 3.4 1.9 10 1.53.8 2.3 11 1.5 4.2 2.7As the ball diameter (and, correspondingly, the diameter of the cavity)increases, the difference in flow rate required to close the valve withand without air bubbles adhered to the ball increases. Therefore, asmall cavity diameter and ball diameter are required to obtain valveclosure flow rates (with and without air bubbles) which are reasonablysimilar in magnitude.

Pressure drop in the valve, due to the difference in the diameters ofthe cavity and of the seat of the valve, as a function of flow rate,also affects system performance. Generally, as the diameter of the ballincreases, or the diameter of the cavity or orifice through the seatdecreases, the pressure drop across the valve increases. Also,increasing the diameter of the ball increases the surface area availablefor adherence of air bubbles, and more air bubbles lowers the effectivedensity of the ball. Therefore, the valve design represents a tradeoffbetween an acceptable pressure drop and satisfactory valve performance(i.e., the flow rate at which the valve actuates).

Thus, to achieve optimum valve characteristics, a compromise betweenpressure drop and closure flow rate should be achieved. The embodimentsshown in FIGS. 5 and 6 are examples of valves in which these variableshave been optimized for a typical residential installation usingstandard ¾ inch water pipe. In these embodiments, the cavity diameter isabout 16 mm (about 0.63 inches), the seat diameter is about 9 mm (about0.35 inches), and the seat angle is about 45° with respect to thelongitudinal axis of the valve, the ball material is Delrin™, with adensity of about 1400 kg/m³, and a diameter of about 11 mm (i.e., about5 mm less than the diameter of the cavity). Of course, these dimensionscan be adjusted to suit any application and desired closure flow rate.Other ball materials with a density of about 1200 kg/m³ to about 1600kg/m³ are suitable, such as Teflon™, with a density of about 1550 kg/m³.Note that the embodiment shown in FIG. 6 is configured as a “T” joint,the third port 38 for connection to a mains fluid supply. Neither of theembodiments of FIGS. 5 and 6 show a provision for capturing (retaining)the ball within the cavity. However, such embodiments are also providedby the invention, as discussed above.

In another embodiment, the back-flushing valve provides bypass flow;that is, flow through the valve when the ball is seated (i.e., when thevalve is in the closed state), to reduce the pressure differenceoccurring across the valve in the closed state. The bypass flow reducesany mechanical shock caused by the valve closing and eliminates pressurewaves that could result in “water hammer” in the associated system. Thislatter situation is most likely to occur in cases where the valve islocated in close proximity to the mains water inlet. For example, in atypical solar domestic hot water system with a mains pressure of about40 pounds per square inch (PSI), a bypass flow rate of about 1% to about20%, preferably about 2% to about 10%, more preferably about 5% to about8%, of the nominal hot water draw flow rate, is suitable. It will beappreciated that when the bypass flow rate is a small percentage of thehot water draw flow rate, the bypass flow will have only a minor effecton the temperature of hot water supplied from the storage tank. However,as the bypass flow rate increases, cooling of the hot water drawn fromthe storage tank will be more significant. Therefore, it is preferableto keep the bypass flow rate as low as possible.

The bypass flow can be provided, for example, by suitable ridges orgrooves cast or machined into the valve, on or near the valve seat. Inone embodiment, shown in FIG. 7, the bypass flow is provided by a smallhole 40 (e.g., about 1 to 1.5 mm in diameter) bored through the seat ofthe valve 26, in a direction substantially parallel to the direction offluid flow. It will be appreciated that such bypass hole allows a smallamount of fluid flow through the valve when the ball 28 is seated. Forexample, at a mains pressure of about 40 pounds per square inch (PSI),the bypass hole provides a bypass flow rate of about 5% to about 10% ofthe nominal hot water draw flow rate. Such percentage of the flow willhave only a minor effect of the temperature of hot water supplied fromthe storage tank; for example, less than 4° C. for a 1 mm bypass hole at40 PSI. Also shown in FIG. 7 is a retaining screw 42 for retaining theball 28 in the valve.

In another embodiment, the back-flushing control valve is inverted andinstalled at the top of the storage tank. In this configuration, thevalve seat 26 and output port 24 are oriented downwards. Accordingly,the ball density is selected so that it generally floats at flow ratesup to about 1.5 L/min to about 2.5 L/min, but is driven down to engagethe valve seat at higher flow rates.

The invention is further described by way of the following non-limitingexamples.

WORKING EXAMPLES Example 1 Experimental Evaluation of a PassiveBack-Flushing Heat Exchanger

To investigate the operation of the passive back-flushing valve, a testapparatus was constructed with two parallel natural convection loops,each loop with a heat exchanger, as described above, connected to a 450L storage tank. The loops were identical except that a passiveback-flushing control valve according to the invention was installed inone of the loops and the other loop was operated without the benefit ofthe valve. The primary side of each heat exchanger was supplied with hotwater (60° C.) from the same source in such a way that the same flowrates and inlet temperatures were maintained in both loops. For the loopwith the back-flushing valve, water was drawn from the bottom of thestorage tank and used to back-flush the heat exchanger at one-hourintervals for 3 to 4 minutes. Flow circulation through the secondaryside of both heat exchangers was driven by natural convection, caused bythe density difference that existed between the heated water in the heatexchange loops and the cooler water in the storage tank. To acceleratethe fouling test, the tank water was initially saturated with CaCO₃,CaSiO₃ and Mg(OH)₂. During the 5-month test period, 68.5 g of CaCO₃ wasadded at three intervals. This quantity ensured that the tank water wasa saturated CaCO₃ solution.

After 5 months of continuous testing the system was stopped and bothheat exchangers were examined. Observation of the heat exchanger withthe passive back-flushing valve revealed that the flow passages of thesecondary side of the heat exchanger were clear and free of any residueor blockage. In addition, the pressure drop across the heat exchangerand associated temperatures were consistent with those observed at thestart of the test. When the heat exchanger from the loop without thepassive back-flushing valve was removed, a large quantity of solidprecipitate was found in the secondary side of the heat exchanger, suchthat the flow passages were effectively blocked. Consistent with thissituation, the pressure drop across the secondary side of the heatexchanger was observed to be much greater than at the beginning of thetest, and greater than that of the back-flushed heat exchanger. A reviewof the temperature data revealed that the rate of heat transfer crossthe heat exchanger without the passive back-flushing valve wassignificantly reduced.

Therefore, the results indicate that under identical conditions of waterconstituents and temperature, the heat exchanger without back-flushingbecame significantly fouled with precipitated CaCO₃, while the heatexchanger fitted with the passive back-flushing valve of the inventionhad no significant amount of fouling and continued operate as initiallyinstalled.

Example 2 Operation of a Passive Back-Flushing System

To illustrate the operation of a passive back-flushing system accordingto the invention, a typical application (i.e., a solar domestic hotwater system (SDHW)) was outfitted with temperature sensors to recordfluid temperatures during a typical hot water draw. As shown in FIG. 8,the system consisted of a storage tank 8 for storing heated water, anatural convection flow loop connected from the bottom of the storagetank to the top of the storage tank through the secondary side of a heatexchanger 4, a mains water inlet “T” 10 and a passive back-flushingvalve 12 similar to that shown in FIG. 5. The fluid entering the primaryside of the heat exchanger was heated to a nominal temperature of about70° C. by solar collectors 2. During the test, temperatures weremeasured at specific locations on the secondary side of the heatexchanger loop (designated T1 to T4). Data shown in FIG. 9 was recordedover a period of time consisting of time intervals before, during andafter the draw of hot water from the top of the storage tank via outlet16. In FIG. 8, direction arrows are shown for the draw mode.

Prior to drawing hot water from the system, the temperature at T1indicated that cool water from the bottom of the storage tank enteredthe secondary side of heat exchanger at 16° C. and was heated to 47° C.The nominal flow through this side of the heat exchanger was 1 L/min atthis time and was caused by buoyancy forces resulting from the heatingof the water (e.g., natural convection). At the start of the draw, hotwater was removed from the top of the storage tank, which caused cold“mains” water to enter through the “T” 10 located above the heatexchanger in the secondary loop. The flow of water into the system atthis point caused the back-flushing valve 12 to close and the flowdirection to reverse, sending cold mains water through the heatexchanger in the opposite direction to that occurring during heating(prior to the draw). The flow of mains water through the system wasabout 3.5 times that of the natural convection flow which occurred inthe heating mode. During the draw period the temperatures below the “T”and the back-flushing valve (T2 and T3) were reduced to the mains watertemperature, due to the inflow of cold water. The temperature at theinlet from the storage tank to the heat exchanger (T1), which was coldprior to the draw, increased slightly during the draw (i.e., duringback-flushing of the heat exchanger). This increase in temperatureoccurred as result of heating of the mains water as it flowed throughthe heat exchanger in the opposite direction to the heating mode. Thetemperature rise of the mains water is lower than that for the naturalconvection driven flow because of the greater flow rate of the mainswater through the heat exchanger.

The data shown in FIG. 9 corresponds to a draw period of approximately 3minutes. At the end of the draw period, removal of water from the systemwas stopped and the flow of mains water into the system ceased. At thispoint, the back-flushing valve re-opened and the natural convection flowof water from the storage tank resumed. The data indicate that about 90%of the convective flow was re-established within 30 seconds. Thisre-establishment of the natural convection flow resulted in a smalldepression of the temperature at the top of the secondary loop (T4) asthe cooler water, below the back-flushing valve, was pushed up out ofthe convective flow loop and into the storage tank.

The contents of all cited documents and Appendix 1 are incorporatedherein by reference in their entirety.

Those skilled in the art will recognize, or be able to ascertain usingroutine experimentation, variations of the embodiments and examplesdescribed herein. Such variations are intended to be within the scope ofthe invention and are covered by the appended claims.

1-35. (canceled)
 36. A module for use with a thermal energy system,comprising: a heat exchanger for transferring thermal energy between asource and a load, the heat exchanger having a primary side associatedwith the source, and a secondary side for receiving fluid to be heatedand outputting heated fluid, the fluid flowing through the secondaryside of the heat exchanger in a first direction; an input for receivingmains fluid; and means for back-flushing the secondary side of the heatexchanger; wherein the means for back-flushing the secondary side of theheat exchanger routinely directs mains fluid through the secondary sideof the heat exchanger in a second direction opposite to the firstdirection without user intervention.
 37. The module of claim 36, whereinthe means for back flushing the secondary side of the heat exchangerdirects mains fluid through the secondary side of the heat exchanger inresponse to a change in at least one variable of the heated fluid. 38.The module of claim 36, wherein the means for back flushing thesecondary side of the heat exchanger comprises a back-flushing valve.39. The module of claim 36, wherein the fluid is water.
 40. The moduleof claim 37, wherein the variable of the heated fluid is at least one offlow rate, temperature, and pressure.
 41. The module of claim 37,wherein the variable of the heated fluid is flow rate.
 42. The module ofclaim 37, wherein the variable of the heated fluid is temperature. 43.The module of claim 36, further comprising at least one pump formaintaining flow of the fluid through the secondary side of the heatexchanger in the first direction.
 44. The module of claim 36, whereinthe load comprises at least one storage tank.
 45. The module of claim36, wherein the source comprises at least one heat source selected fromsolar heat, waste heat, geothermal heat, industrial process heat, a heatpump, a boiler, and a furnace
 46. The module of claim 36, wherein thesource comprises at least one solar collector.
 47. A solar thermalenergy system, comprising: the module of claim 36; at least one solarcollector; and at least one storage tank associated with the load.
 48. Amethod for preventing fouling of the secondary side of a heat exchangerin a thermal energy system, comprising: providing a heat exchanger thattransfers thermal energy between a source and a load, the heat exchangerhaving a primary side associated with the source and a secondary sidefor receiving fluid to be heated and outputting heated fluid, the fluidflowing through the secondary side of the heat exchanger in a firstdirection; providing a source of mains fluid; and routinely directingmains fluid through the secondary side of the heat exchanger in a seconddirection opposite to the first direction without user intervention. 49.The method of claim 48, wherein directing mains fluid through thesecondary side of the heat exchanger in a second direction opposite tothe first direction comprises directing mains fluid in response to achange in at least one variable of the heated fluid.
 50. The method ofclaim 48, wherein the fluid is water.
 51. The method of claim 49,wherein the variable of the heated fluid is at least one of flow rate,temperature, and pressure.
 52. The method of claim 49, wherein thevariable of the heated fluid is flow rate.
 53. The method of claim 49,wherein the variable of the heated fluid is temperature.
 54. The methodof claim 48, wherein the load comprises at least one storage tank. 55.The method of claim 48, wherein the source comprises at least one heatsource selected from solar heat, waste heat, geothermal heat, industrialprocess heat, a heat pump, a boiler, and a furnace
 56. The method ofclaim 48, wherein the source comprises at least one solar collector.